Non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, have been used widely as the batteries to be used for personal computers, cellular phones, personal digital assistant devices, etc. due to their high energy densities.
Such non-aqueous electrolyte secondary batteries typified by lithium secondary batteries are high in energy density and they may generate heat due to the occurrence of high current when internal short-circuit or external short-circuit has been caused, for example, by damage to a battery or damage to an instrument in which a battery is used. Therefore, non-aqueous electrolyte secondary batteries have been demanded to prevent generation of a certain amount or more of heat and secure high safety.
A method of imparting a shutdown function to prevent further heat generation by blocking the passage of ions between the positive and negative electrodes with a separator at the time of abnormal heat generation is common as means for securing safety. In a battery including this separator, the separator melts and closes pores on abnormal heat generation to block the passage of ions and thereby can suppress further generation of heat.
As such a separator having a shutdown function, a porous film made of a polyolefin (hereinafter sometimes referred to as a “porous polyolefin film”) is used, for example. A separator made of the porous polyolefin film melts and closes pores at about 80 to 180° C. on abnormal heat generation of a battery to block (shut-down) the passage of ions and thereby suppress further generation of heat. However, in some cases, a separator made of a porous polyolefin film allows a positive electrode and a negative electrode to come into direct contact with each other due to the shrinkage, rupture, or the like thereof, resulting in the occurrence of short circuit. A separator made of a porous polyolefin film has insufficient shape stability and may be unable to suppress abnormal heat generation caused by short circuit.
On the other hand, there has been studied a method of imparting shape stability at elevated temperatures to a separator by laminating a porous film made of a heat-resistant material (hereinafter sometimes referred to as a “heat-resistant layer”) to a porous polyolefin film. As such a highly heat-resistant separator, for example, a separator prepared by immersing a regenerated cellulose film in an organic solvent to render it porous, followed by lamination to a porous film as a substrate (hereinafter sometimes referred to as a “porous film substrate”), and a laminated porous film prepared by applying a coating slurry containing a particulate filler, a water-soluble polymer, and water to the surface of a porous film substrate, followed by drying have been proposed (see, for example, Patent Documents 1 and 2).
Although such a laminated porous film is produced by applying a coating slurry containing an inorganic filler and a binder resin to the surface of a porous film substrate uniformly, if the coating slurry penetrates into the porous film substrate during the application step, the binder resin, which is one of the components of the coating slurry, penetrates into the porous film substrate. Therefore, there is a problem that the inherent properties of the porous film substrate become no longer possible to be maintained, for example, the ion permeability or the shutdown property of the porous film substrate deteriorates.
Moreover, a porous film substrate for a laminated porous film is preferred to have a high porosity (for example, 50% or more) in order to gain improved ion permeability when used as a separator. In the porous film substrate, however, when the coating slurry has penetrated into the porous film substrate during the above-mentioned application step, the porous film substrate shrinks due to the shrinkage stress produced when the solvent component in the coating slurry that has penetrated is vaporized, so that the porous film substrate becomes incapable of maintaining a high porosity. Therefore, the porous film substrate has a problem that the characteristics of a resulting laminated porous film become inferior to those expected from the inherent characteristics of the porous film substrate.